This invention relates to a process for the manufacture of cyanuric acid by heating urea at a temperature above its melting point, and recycling cyanuric acid granules, with urea adsorbed thereon in a heat exchange zone and heating to a predetermined temperature.
It is known in the prior art to manufacture cyanuric acid by heating urea to a predetermined temperature above its melting point. Cyanuric acid, i.e., C.sub.3 N.sub.3 O.sub.3 H.sub.3, exists in two tautomeric forms as follows: ##STR1## The thermal decomposition of urea into cyanuric acid in either form follows the reaction EQU 30=C(NH.sub.2).sub.2 .fwdarw.C.sub.3 N.sub.3 O.sub.3 H.sub.3 +3NH.sub.3 .
Other side reactions also occur and result in the formation of ammelide, amidine and guanidines.
The prior art manufacture of cyanuric acid from urea based upon the above-discussed reaction is generally conducted in two stages. The first stage involves taking molten urea, or urea in aqueous solution, adsorbed on granules of crude cyanuric acid which had been prepared beforehand and heating to a temperature of 220.degree.-280.degree. C. The granules impregnated with urea are recycled numerous times, typically 5-9, to achieve maximum conversion of urea for production of cyanuric acid.
The second stage of the process is a purification of the crude cyanuric acid by treating with a strong acid, in dilute solution, such as sulfuric acid, phosphoric acid, or preferably, nitric acid.
This type of prior art process is generally described in French Pat. Nos. 1,183,672 and 1,229,102 wherein cyanuric acid granules impregnated with sprayed urea are passed through a heat exchange zone such as rotary furnace, or trough containing paddles, or an endless screw, or other conveying means. Heat of reaction is supplied externally by, for example, radiant heat panels or natural gas burners. A portion of the stream is then recycled into the furnace for further treatment. Typically, the crude cyanuric acid is taken up in a conveyor and directed into a hopper with a sieve, in which fine granules are separated from coarse granules and the coarse granules ground into fine granules. By fine granules is meant granules no greater than 12.5 mm, with particles larger than this being considered coarse granules, more preferably, the fine granules are 1.9-12.5 mm in size. The fine granules and the ground coarse granules are then reintroduced into the top of the zone.
However, a problem with this prior art method is that if the cyanuric acid, which is at a temperature of about 220.degree. C., is directly recycled, the zone surfaces foul up rapidly and the installation is brought to a standstill.
Previously, this problem has been solved by cooling the granules and spraying with water before recycling. Typically, about 2000 kg/hr of cyanuric acid is treated with 80 to 100 l/day of sprayed water before recycling to lower the temperature thereof to about 180.degree. C. The water spraying improves the adsorption capacity of the granules to the urea solution. More specifically, without water spraying the porosity of the granules, measured by the mercury porosity standard, is 12-15% as compared to with water spraying wherein a porosity of about 27% is achieved.
On the other hand, the addition of water results in a number of disadvantages. More specifically, energy consumption increases because of lowering of the temperature by the water, as well as the fact that the water must subsequently be removed. Furthermore, condensation occurs in the hoppers used for sieving, and in one attempt to solve this problem of condensation the covers of the hoppers were opened, but this resulted in a high emission of irritant dusts.